Disorders of perception, attention, and memory frequently accompany the major mental diseases. To begin to understand the neural mechanisms of these mental processes, we are recording the activity of neurons in the extrastriate cortex of monkeys engaged in tasks requiring visual discrimination, selective attention, and recognition memory. We have found in area V4 of the occipital cortex and area TE of the inferior temporal cortex that selective attention gates visual processing by filtering unwanted information from the receptive fields. Even the degree to which attended stimuli are processed in these areas depends on "how much" attention or effort is devoted to them. Thus, the information-processing capacity of cortical neurons depends not only on hard-wired mechanisms but on cognitive state. To identify the mechanisms by which cognitive state modulates cortical activity, we have begun to examine both extrastriate neuronal activity and animal behavior in an attention-demanding task following lesions of selective portions of the prefrontal cortex, posterior parietal cortex, limbic- system, and basal ganglia.